1. Field of the Invention
The present invention relates to a grain oriented silicon steel sheet having an excellent primary glass film and magnetic properties.
2. Description of the Prior Art
A grain oriented silicon steel sheet has been used as a magnetic core material of electrical equipment and is required to have a low iron loss. Conventionaly, many improvements contributing to a lowering of the iron loss value have been conducted.
It is known that creating tension to the steel sheet is useful, and lowers the iron loss of the grain oriented silicon steel sheet.
To induce tension to the steel sheet, it is useful to form a primary glass film (hereinafter refferred to as "primaly film") constituted of a material having a coefficient of thermal expansion lower than the steel.
The primary film, i.e., oxide film, is formed which an oxide on the surface of the steel sheet reacts with an annealing separator (a separating agent to prevent sticking among the steel sheets during annealing) and can induce tension in the steel sheet, and this primary film has excellent adhesion.
The conventional process for the formation of an insulation primary film used in the art comprises subjecting a silicon steel strip having a final sheet thickness to a primary recrystallization annealing involving decarbonization, coating the annealed strip with a slurry of magnesia (MgO) as an annealing separator (i.e., a separating agent to prevent sticking among the steel sheets during annealing), drying the coating strip, winding the dried strip to form a strip coil and the final annealing for coil in the secondary recrystallization. During this secondary recrystallization annealing step, it causes the Si or silicon oxide in the steel sheet to react with the annealing separator, to thus form a ceramic insulating primary film called "forsterite" (Mg.sub.2 SiO.sub.4).
The forsterite imparts tension to the steel sheet, to thus reduce (improve) the iron loss of the grain oriented silicon steel sheet, and therefore, the state of the forsterite formed on the surface of the grain oriented silicon steel sheet plays an important role in the growing of a grain having a {110} &lt;001&gt; (Goss) orientation in the secondary recrystallization annealing step. Specifically, when raising the temperature in the secondary recrystallization annealing step, and when a secondary recrystallization is conducted without the formation of a sufficiently dense primary film, a fine nitride or sulfide serving as an inhibitor comes off as it is or through decomposition in an early stage, so that, upon a temperature rise, the amount of inhibitor that preferentially grows grains having a Goss orientation and inhibits the grain growth having another orientation decreases and provides a product having very poor magnetic properties and a texture called a "fine grain" wherein grains having a Goss orientation grow only in part or do not grow at all. In order to form a dense primary film, an attempt has also been made to coat a steel sheet (strip) with the annealing separator comprising magnesia (MgO), and added thereto, titanium oxide (TiO.sub.2 or the like) or another compound.
Whether, the forsterite is formed in a rather low temperature range, depends significantly on the selection of the annealing separator. The annealing separator consists mainly of MgO in most of the proportion, TiO.sub.2, boric or boride base compounds and sulfide or sulfuric base compounds, etc. Especially boride or boric base or sulfide or sulfuric base compounds are necessary. When adding these compounds to MgO and TiO.sub.2, both MgO and SiO.sub.2, which were formed in the primary recrystallization annealing react even in a rather low temperature range of the secondary recrystallization annealing and also enable forsterite to form in the low temperature range.
Gas composition in the secondary recrystallization annealing is another important factor that affects the primary film formation. For example, moderate oxidizing atmosphere at 600.degree.-850.degree. C. in the annealing (Japanese Patent Application No. 1-91956) or such a method that the sum of nitrogen and Ar gas pressure ratio is 30% or more under nitrogen gas equal and more of 25% and remnants covered with hydrogen gas in the temperature range up to 800.degree.-850.degree. C. (Japanese Patent Application No. 2-301919), are also effective.
Furthermore, as shown in a patent of Japanese Unexamined Patent Publication No. 48-39338, a tension in the silicon steel sheet that can be obtained by a method of coating a compound of colloidal silica and phosphate on the steel sheet followed by baking, is efficient to decrease the iron loss (core loss). Therefore, such a treatment as tension providing insulation coating (secondary coating) on the steel sheet with primary film formed in the secondary recrystallization annealing, is one of the most conventional techniques in the grain oriented silicon steel sheet production. In this case, however, if a thickness of the insulating coating film is increased too much, to obtain a lot of tension in the steel sheet, the spacing factor decreases, but also adhesion of the coating film deteriorates.
On the other hand, methods other than these such as a method to obtain a lot of tension in the steel sheet as dry coating TiN or TiC on the sheet by way of CVD (Chemical Vapor Deposition) or PVD (Physical Vapor Deposition) are invented in the patents of Japanese Examined Patent Publication No. 63-35684 and No. 63-35685. These methods have rather effective tension in the sheet. But they require higher production cost because of the high degree of vacuum facilities and the long treatment time.
In these prior art techniques, it is not easy to stably form a satisfactory primary film on the surface of a silicon steel sheet. This is attributable to the fact that the elucidation of the mechanism for the formation of the primary film is not always sufficient. Furthermore, since no sufficient elucidation of the relationship between the structure of the primary film and the production conditions has been made, the formation of the primary film is conducted mainly based on experience, so that the formation of primary film cannot be freely controlled.